In a conventional transmitter apparatus in a radio communication system, nonlinear distortion that occurs as a result of amplifying a signal in a power amplifier is compensated and usage efficiency of the power amplifier is enhanced by using the power amplifier in a saturation region.
FIG. 1 illustrates a distortion compensating apparatus 100 to compensate a characteristic of a nonlinear circuit that includes a power amplifier 200 by using a feedback loop. The distortion compensating apparatus 100 includes an adaptive distortion compensating algorithm processing unit 110 and a multiplier 120.
FIG. 2A is a graph illustrating input/output power characteristics of the power amplifier 200 illustrated in FIG. 1. The graph illustrates a linear region A1 where input power and output power exhibit a proportional characteristic and a nonlinear region A2 (indicated with a solid line) where output power approaches a saturation state.
The adaptive distortion compensating algorithm processing unit 110 receives a transmission signal 300 serving as a reference signal and part of a radio frequency (RF) output signal 340 of the power amplifier 200 serving as a feedback signal 350, and calculates a distortion compensating coefficient 320 by operating an adaptive algorithm so as to minimize the error between those signals. The multiplier 120 supplies a signal 330, which is generated by multiplying the transmission signal 300 by the distortion compensating coefficient 320, to an input terminal of the power amplifier 200.
That is, the distortion compensating apparatus 100 compensates nonlinear distortion of the power amplifier 200 by multiplying the transmission signal 300 by the distortion compensating coefficient 320 in advance and then supplying the signal 330 generated through the multiplication to the power amplifier 200. As a result, usage efficiency can be enhanced by using the power amplifier 200 in the region “A2” illustrated in FIG. 2A.
FIG. 2B illustrates an effect of the distortion compensation. The horizontal axis indicates frequency, whereas the vertical axis indicates amplitude (radiation power or voltage). The frequency characteristic of radiation power before distortion compensation is indicated with a solid line. Radiation powers B1 and B2 are generated in bands C1 and C3 outside a band C2 of an input signal. The radiation power levels in the bands C1 and C3 after distortion compensation (broken lines) are decreased to radiation power levels B1A and B2A.
In the above-described conventional distortion compensating apparatus, an influence of a frequency characteristic of an analog circuit other than nonlinear distortion needs to be reduced in order to prevent impairment of an effect of distortion compensation. Such an influence can be reduced to some extent by performing equalization using a filter having a reverse characteristic of the frequency characteristic of the analog circuit including the power amplifier 200.
However, the characteristics of the analog circuit significantly varies due to change in temperature or over time, and thus an adaptive equalizer is required. In this point of view, the applicant of the present application has suggested a distortion compensating apparatus to adaptively compensate a nonlinear characteristics of a circuit having nonlinear distortion (Japanese Laid-open Patent Application Publication No. 2003-298362, hereinafter referred to as Patent Document 1).
FIG. 3 is a block diagram illustrating a basic configuration of the distortion compensating apparatus 100 disclosed in Patent Document 1.
Referring to FIG. 3, the distortion compensating apparatus 100 includes an adaptive distortion compensating unit 100A to compensate nonlinear distortion by controlling an input signal of a power amplifier 200 by using an adaptive algorithm so that an error between a reference signal which is a transmission signal 300 and a feedback signal 350 from the power amplifier 200 having nonlinear distortion becomes small.
Furthermore, the distortion compensating apparatus 100 includes an adaptive equalizer 100B provided between the adaptive distortion compensating unit 100A and the power amplifier 200 or connected to the front part of the adaptive distortion compensating unit 100A.
The adaptive equalizer 100B includes a digital filter 130, a filter coefficient group holding memory 150 to hold in advance filter coefficients some of which are to be set to the digital filter 130, and an adaptive equalization processing unit 140 to adaptively select a filter coefficient for reducing out-of-band radiation power of a feedback signal from the filter coefficient group and to set the selected filter coefficient to the digital filter 130.
According to the invention described in Patent Document 1, part of an RF output signal 340 from the power amplifier 200 is input as the feedback signal 350, the average out-of-band radiation power is measured by the adaptive equalization processing unit 140, a coefficient value held in advance in the filter coefficient group holding memory (hereinafter referred to simply as “memory”) 150 is selected and read by using a measurement value obtained from the measurement as an address, and the coefficient value is set to the digital filter 130.
Here, in the selection of an equalizer filter through measurement of out-of-band radiation power in Patent Document 1, that is, in the method for selecting a coefficient for the digital filter 130, analysis based on Fourier transform is performed to measure out-of-band radiation power by the adaptive equalization processing unit 140.
For this reason, the number of calculations increases and longer time is required to select a filter coefficient disadvantageously. Furthermore, there are unsolved problems associated with the poor track ability for variations in power of the transmission signal 300 is poor, and with the need to change a measurement point of out-of-band radiation power when band variation occurs.